Hi Jon.
Sounds like you are a boat fixer. Nice hobby.
The Kittiwake boats are reported to be built by Kenner Boat Company.
You do not mention the length of the boat. Kenner built a number of boats. In 66 the Kittiwake was a new boat for the company. It would be difficult too guess the core thickness as some boat builders used plywood, others Balsa, and some just laid up fibber glass. You are likely to find that the deck varies in thickness depending where you are on the boat.
Founded by Bill and Dick Kenner. Since the late 1950’s they had been building small fiberglass runabouts under the ‘Kencraft’ name. With the introduction of the ‘Ski Barge’, Kenner Boat Company experienced it’s most significant growth and established a reputation as a leader in the field. By the...
sailboatdata.com
I find that true on my boat. Some area are 1/4" core others have 1/2" plywood.
So that I do not have to drill a hole, I look for a thru bolt, extract it and then inspect the deck thickness both glass and core if there is any.